This invention relates to engine starters, and more particularly to pinion shifting mechanisms of an automotive engine starter for translating the pinion into meshing engagement with the ring gear of the engine.
Generally, automotive internal combustion engines comprise engine starters driven by an electric motor. FIG. 1 shows a typical structure of an automotive engine starter 1 which comprises four main portions: a dc electric motor 2; a pinion assembly 3 mounted slidably on the output shaft 2b of the motor 2; an electromagnetic switch device 4 disposed at a side of the electric motor 2; and a shift lever 5 operatively coupling the plunger or armature of the electromagnetic switch device 4 with the pinion assembly 3. Let us describe these portions in the above order.
The armature shaft 2a of the electric motor 2 has an extension (i.e. the output shaft) 2b extending forward (toward right in the figure) from the motor 2. The pinion assembly 3 slidably mounted on the output shaft 2b of the motor 2 comprises a unidirectional or one-way clutch 18 consisting of a clutch outer member 18a, an inner member 18b, and rollers 18c disposed between the outer and inner members 18a and 18b, wherein the pinion 19 for engaging with the ring gear of the engine (not shown) forms a forward extension of the clutch inner member 18b. The rear cylindrical extension 20 of the clutch outer member 18b is splined at its inner surface to the outer surface of the output shaft 2b on which helical splines 2c are formed. Thus, the pinion assembly 3 is slidable on the output shaft 2b in the axial direction, transmitting the torque of motor 2 unidirectionally via the one-way clutch 18. Further, on the outer surface of the rear cylindrical extension 20 is secured a stop disk 21 so that the bifurcate lower end potion 5b of the shift lever 5 extends between the stop disk 21 and the rear surface of the clutch outer member 18a to engage with the pinion assembly 3 thereat.
The electromagnetic switch device 4, for driving and shifting the pinion assembly 3 and for making the energization current of the motor 2, comprises a stationary core and frame assembly and an armature or plunger assembly disposed slidably therein. First, let us describe the stationary portion:
The cylindrical outer frame 6 comprises an annular front end wall 6a through which the plunger 7 extends. An annular disk-shaped stationary core 8, having an inner cylindrical extension opposing the annular rear end surface of the plunger 7 across a gap g, is secured to the rear end of the frame 6 to form together therewith a frame assembly within which an excitation coil 10 wound on a coil bobbin 9 is accomodated. Further, the rear end of the frame assembly is covered by a cap 13 of synthetic resin, through which extends a terminal bolt 14 having a stationary contact point 14a at the front end thereof.
On the other hand, the plunger assembly comprises the cylindrical plunger 7 which, when the coil 10 is energized, is attracted toward the core 8 to slide within the inner surface of the coil bobbin 9. The plunger 7 has a rod-shaped rear extension (plunger rod) 7a extending through the central aperture of the core 8; on the plunger rod is mounted an annular disk-shaped movable contact 12 via a support sleeve 12a. The sleeve 12a is slidable on the rod 7a, and is urged by a helical spring 24 toward the stop ring 7c secured on the rod 7a; thus, the movable contact 12 yields and slides on the rod 7a by a predetermined small axial length when the plunger 7 is translated to its extreme rear position and the contact 12 comes into contact with the stationary contact 14a. Further, the plunger 7 has a central cylindrical bore 7b formed therein to open toward forward direction; within this bore 7b extends axially slidably a piston-like lever engager member 15 having a flange 15a at the rear end thereof. The front extension of the engager member 15 extends forward from the front end of the bore 7b through an annular member 16 closing the bore 7b, and a pair of engagement disks 15b secured to the front extension of the member 15 engage with the bifurcate upper end of the lever 5. Further, a helical urging spring 17, disposed around the engager member 15 within the bore 7b of the plunger 7 to bear on the annular member 16 secured to the plunger 7 at one end and on the flange member 15a of the engager 15 at the other, urges the member 15 deeper into the bore 7b. Further, a helical return spring 11 is disposed between the plunger 7 and the core 8 to bear on the rear end of the plunger 7 and the inner radial extension of the core 8; the return spring 11 urges the plunger 7 in the direction away from the core 8.
The lever 5 having upper and lower engaging end portions 5a and 5b is pivoted on the pivot support 5c situated substantially at the middle of the length of the lever 5. The pivot support 5c is held between a support surface 22a of the front bracket 22 of the starter 1 and a metal plate 23 secured on a support base 23a. Thus, the lever 5 is rotatable on the pivot support 5c.
Let us now describe briefly the operation of the starter 1. When the key switch (not shown) of the automobile is turned on, an excitation current is supplied to the coil 10 of the electromagnetic device 4; thus, the resulting magnetic force attracts the plunger 7 toward the core 8. The movement of the plunger 7 is transmitted, via the engager member 15 and the lever 5, to the pinion assembly 3; thus, the lever 5 rotates, in the counter-clockwise direction in the figure, until the side of the pinion 19 abuts against the sides of the teeth of the ring gear of the automotive engine (not shown) to stop the rotation of the lever 5. When the rotation of the lever 5 is stopped, the movement of the engager member 15 engaging with the lever 5 is also stopped. The plunger 7, however, continues to translate further toward the core 8, thereby compressing the spring 17; thus, the compressed spring 17 urges, via the engager member 15 and the lever 17, the pinion 19 against the ring gear of the engine. Finally, the movable contact 12 comes into contact with the stationary contact 14a to make the energization current supplied to the electric motor 2. The ensuing torque of the output shaft 2b of the motor 2 is transmitted via the one-way clutch 18 to the pinion 19. Thus, the pinion 19 rotates to a teeth-to-gap position (i.e. meshing angle) with respect to the ring gear of the engine; as a result, the pinion 19, thanks to the urging force of the compressed spring 17 transmitted via the lever 5, is translated axially forward into a fully meshing engagement with the ring gear of the engine.
Thus, the elecromagnetic switch device 4 has two-fold functions: first, it functions as a relay for making the energization current supplied to the motor 2; second, it drives and shifts, via the rotation of the lever 5, the pinion assembly 3 into engagement with the ring gear of the engine.
The pinion shifting mechanism of the above described starter, however, suffers from the disadvantage that it is large-sized and expensive. This is deemed in the main due to the following factors:
(1) In order to ensure that the pinion 19 is forced into secure engagement with the ring gear of the engine, the urging spring 17 must be capable, when compressed, of exerting a force strong enough to compel the pinion 19 into engagement with the ring gear. The urging force of the compressed spring 17, however, also acts against the movement of the plunger 7 toward the core 8 when the rotation of the lever 5 is stopped. In order to overcome the strong force of the urging spring 17 against the movement of the plunger 7, the coil 10 must be capable of developing a magnetic force strong enough to attract the plunger 7 toward the core 8 against the urging force of the spring 17. Thus, a large-sized coil 10 with a large amount of expensive copper is necessary.
(2) For the purpose of ensuring a reliable operation of the starter device, a considerable amount of clearance is needed with respect to the engangements of the lever 5 with the engager member 15 on the one hand and with the one-way clutch 18 on the other; namely, a certain amount of clearance must be provided between the upper end 5a of the lever 5 and opposing surfaces of the engagement disks 15b secured to the engager member 15; the same is true with respect to the clearance between the lower end 5b of the lever 5 and the opposing surfaces of the stop disk 21 and the clutch outer member 18a. (The reason therefor is made clear below in the description of the preferred embodiments.) In order to compensate for this large clearance provided for the engagement of the lever 5 at the upper and lower ends thereof, the total length of the gap g between the plunger 7 and the core 8 travelled by the plunger 7 from its initial front to final rear position must be made longer. This results in further increase in the size of the device.
(3) The length of the upper arm of the lever 5 from the pivot support 5c to the upper end 5a thereof also necessitates that the plunger 7 moves over a large gap length g. This is an additional factor for increasing the size of the starter device.
Of the above three factors which contribute to the increase in the size and cost of the starter, let us now explain the first mentioned one in some detail by reference to FIG. 2. FIG. 2 shows, in a solid curve P and a dots and dash curve P', the relation between the gap length g (taken along the abscissa) between the plunger 7 and the core 8 and the magnetic attractive force (taken along the ordinate) acting therebeteen; further, the urging force of the compressed spring 17 acting on the plunger 7 when the plunger 7 is at a distance g from the core 8 is shown by the solid curve S. The urging force S begins to act on the plunger 7, in the direction away from the core 8, when the side of the pinion 19 abuts on the side of the ring gear of the engine; thereafter, the urging force S increases linearly with the decrease of the gap length g. On the other hand, the magnetic attracting force P or P' acting on the plugner 7 toward the core 8 is roughly inversely proportional to the gap length g, the magnitude of the force P or P' at respective gap lengths g being determined by the voltage supplied to the excitation coil 10. Generally, the starter device 1 is supplied with an electric power from a storage battery at the rated voltage of 12 V; the solid curve P shows the attractive force acting on the plunger 7 at the rated voltage of 12 V. However, due to various factors such as the temperature rise, the voltage applied across the coil 10 may occasionally be reduced to or below about two thirds (2/3) of the rated value, i.e. to or below about 8 V; in such case, the attracting force acting on the plunger 7 may be reduced to the level shown by the dots and dash curve P', which touches on the curve S of the urging force of the spring 17. If this happens, the attracting force P' acting on the plunger 7 toward the core 8 and the urging force S acting thereon in the direction away from the core 8 is balanced at the gap length g at which the two curves P' and S are tangent to each other; hence, the plunger 7 cannot be moved toward the core 8 beyond that gap length at which the two forces are balanced, with the result that the energization current can never be supplied to the motor 2. Howver, the magnitude of the urgining force S (at respective gap lengths g) of the spring 17 cannot be reduced below a predetermined level, since it must be strong enough to force the pinion 19 into eangagement with the ring gear of the engine; thus, the number of turns of the coil 10 should be selected large enough to ensure that the resulting attracting force P' is substantially above the level of the urging force S over the whole gap length g even when the voltage supplied thereto is reduced to substantially below the rated voltage.
With regard to the second and third factors (2) and (3) mentioned above, we may note the following points which are related to the first factor (1). As shown in FIG. 2, the attractive force P or P' acting on the plunger 7 from the core 8 is roughly inversely proportional to the gap length g. Thus, if the initial gap length g (i.e. the whole distance travelled by the plunger 7) is reduced, the coil 10 of the same size develops a greater attractive force P or P' over the length of axial movement of the plunger 7 from its initial (extreme front) to the final (extreme rear) position. If, on the contrary, the initial gap length is great, the magnitude of the attracting force P or P' is reduced to a very low level near the initial position of the plunger 7; this results in the need for a large-sized excitation coil 10. Thus, the greater initial gap length not only results in larger axial dimension of the electromagnetic switch device 4, but also in a larger and more expensive coil 10 thereof.